1. Field of the Invention
The present invention relates to a servo accelerometer that has a pendulum provided with a coil interlinked with a magnetic field produced by a permanent magnet and is configured to apply a current determined by the amount of swing of the pendulum to the coil to balance the pendulum in the vicinity of the zero point.
2. Description of Related Art
FIGS. 1A and 1B shows a configuration of a conventional servo accelerometer of this type.
A pendulum 12 disposed inside a circular frame 11 has the shape of a circular plate having a part of the circumference cut by a chord. The pendulum 12 is supported by the frame 11 by the cut part 12a of the pendulum 12 being coupled to the frame 11 by a pair of hinges 13. The frame 11, the pendulum 12 and the pair of hinges 13 are integrally formed of quartz glass. The pair of hinges 13 is thin in order to allow a required swing of the pendulum 12.
The opposite surfaces of the frame 11 abut against a first housing 14 and a second housing 15, respectively, and the frame 11 is held between the pair of the first and second housings 14 and 15. Both the first and second housings 14 and 15 substantially have the shape of a cylinder one end of which is open and the other end of which is closed, and the open ends abut against the frame 11. Annular protrusions 14g and 15g are formed along the inner circumference of the open ends of the first and second housings 14 and 15, which abut against the frame 11. The first and second housings 14 and 15 serve also as a magnetic yoke and are made of a magnetic material. The magnetic material may be Invar, which is magnetic and has a low thermal expansion coefficient.
At the center of the inside of the first and second housings 14 and 15, a first permanent magnet 16 and a second permanent magnet 17, both of which are cylindrical samarium-cobalt magnets, are disposed. In this example, the first and second permanent magnets 16 and 17 are installed on the inner surface of closing parts 14a and 15a of the first and second housings 14 and 15 with disk-shaped bottom pole pieces 18 and 19 interposed therebetween, respectively, and disk-shaped pole pieces 21 and 22 having an increased thickness along the circumference are disposed on the upper surface of the first and second permanent magnets 16 and 17, respectively.
The first and second permanent magnets 16 and 17, the bottom pole pieces 18 and 19, and the pole pieces 21 and 22 are assembled by adhesion, for example. The bottom pole pieces 18 and 19 and the pole pieces 21 and 22 are made of an electromagnetic soft iron (compliant with Japanese Industrial Standards C 2503), and the bottom pole pieces 18 and 19 are fixed to the closing parts 14a and 15a of the first and second housings 14 and 15 by adhesion or laser welding, for example. The bottom pole pieces 18 and 19 serve to accommodate the difference in thermal expansion between the first and second housings 14 and 15 and the first and second permanent magnets 16 and 17.
For example, the first permanent magnet 16 is arranged so that the N pole abuts against the pole piece 21, and the S pole abuts against the bottom pole piece 18. The first permanent magnet 16 and the first housing 14 form a primary magnetic circuit, and a first magnetic gap 23 is formed between the inner circumference of the protrusion 14g at the open end of the first housing 14 and the first permanent magnet 16 or, more specifically, the outer circumference of the pole piece 21 in this example. In the second housing 15, a similar second magnetic gap 24 is formed.
In the cylindrical first and second magnetic gaps 23 and 24, a first coil 27 wound around a bobbin 25 and a second coil 28 wound around a bobbin 26 are disposed, respectively. The first and second coils 27 and 28 are coaxial with the first and second permanent magnets 16 and 17 and attached to the opposite surfaces of the pendulum 12. The ends of the bobbins 25 and 26 on the side of the pendulum 12 are closed by attachment plates 25a and 26a, and the first and second coils 27 and 28 are attached to the pendulum 12 by fixing the attachment plates 25a and 26a to the pendulum 12 by adhesion.
In this example, capacitance-type displacement detecting means detects displacement (swing) of the pendulum 12. On the opposite surfaces of the pendulum 12, arc-shaped electrodes 29a and 29b surrounding the first and second coils 27 and 28 are formed by gold plating or the like. The first and second housings 14 and 15 constitute electrodes opposed to the electrodes 29a and 29b. Within the angular range of the electrodes 29a and 29b formed on the pendulum 12, the open end of the first and second housings 14 and 15 has frame abutting surfaces 14b and 15b, recesses 14c and 15c, and electrode surfaces 14d and 15d arranged in this order from the outside, as shown in FIG. 1A. The electrode surfaces 14d and 15d are spaced apart from the electrodes 29a and 29b on the pendulum 12 by a predetermined distance.
The servo accelerometer configured as described above detects the swing of the pendulum 12 caused by an input acceleration in the X direction as a variation in capacitance between the electrodes 29a and 14d and between the electrodes 29b and 15d. The electrode surfaces 14d and 15d are at a common potential, the detection signals on the electrodes 29a and 29b are differentially amplified by a predetermined electric circuit (not shown), and a current determined by the capacitance difference detected on the electrodes 29a and 29b flows through the first and second coils 27 and 28. The current flowing through the first and second coils 27 and 28 and the magnetic field produced by the first and second permanent magnets 16 and 17 interact to restore the pendulum 12 to the original position, and the pendulum 12 is balanced in the vicinity of the zero point. The current flowing in this case is proportional to the acceleration applied to the pendulum 12, so that the input acceleration can be determined from the current.
As described above, for the conventional servo accelerometer, the frame 11, the pendulum 12 and the hinges 13 are made of quartz glass, and the first and second housings 14 and 15 are made of Invar. By using those materials having low thermal expansion coefficients in this way, dimensional changes or displacements caused by temperature changes or strains induced by stresses are minimized.
However, Invar, which is used for the first and second housings 14 and 15 to make the housings participate in the magnetic circuit, has a saturation flux density of about 1.2 T at room temperature (25 degrees C.), and the saturation flux density of Invar highly depends on the temperature. In particular, in a high temperature environment, the magnetic circuit is easily saturated. Thus, the operating temperature range and the measurement range are limited, and the magnetic circuit can be downsized only to a limited extent.
FIGS. 2A and 2B shows a configuration of a servo accelerometer described in Japanese Patent Application Laid-Open No. H8-292208 (issued Nov. 5, 1996, referred to as Patent literature 1 hereinafter) that solves the problem of the saturation of the magnetic circuit described above. In this example, magnetic reinforcing plates 31 and 32 are attached to the outer surface of the closing parts 14a and 15a of the first and second housings 14 and 15, respectively.
The magnetic reinforcing plates 31 and 32 are made of a material having a higher saturation flux density than the first and second housings 14 and 15. The material may be an electromagnetic soft iron, for example.
The magnetic reinforcing plates 31 and 32 disposed on the outer surface of the closing parts 14a and 15a of the first and second housings 14 and 15 serve to eliminate the saturation of the magnetic circuit at the closing parts 14a and 15a. 
However, the servo accelerometer having the magnetic reinforcing plates 31 and 32 shown in FIGS. 2A and 2B cannot be easily downsized because of the magnetic reinforcing plates 31 and 32 attached to the outer surface of the first and second housings 14 and 15. The servo accelerometer further has a problem that the cost also increases because of the increased number of components.